The present invention uses a substrate which, in the preferred embodiment includes a silicon layer. However, a deposited material, such as polysilicon or amorphous silicon, may also be used. Typically, these are semiconductor wafers, although it is possible to use other materials, such as silicon on sapphire (SOS) . Therefore, "wafers" is intended to refer to the substrate on which the inventive tips are formed.
High resolution microscopes play an important role in research by providing an image of objects previously imperceptible. The "scanning probe" microscopes are becoming more important in industry as such microscopes have several advantages over the scanning electron microscopes (SEM) presently used. Some of the advantages include: superior resolution, minimal sample damage, and the scanning probe microscopes provide quantitative, three-dimensional topographic data. The scanning tunneling microscope (STM), and atomic force microscope (AFM) are among the currently available high resolution probe scanning microscopes.
These high resolution microscopes function by means of a cantilever system. A very fine, sharp tip is disposed on one end of a soft cantilever spring on the other end of the cantilever is a mechanism for sensing the cantilever's deflection. A feedback loop monitors and controls the deflection. The microtip is used to scan a surface. A mechanical scanning system moves the surface with respect to the tip in a raster pattern. When the very fine tip encounters a microscopic bump in the surface, the sensing mechanism records the distortion, thereby producing an image of the surface as the surface is scanned on a display system that converts the measured data into an image.
Very fine, extremely sensitive tips are required for clarity in the high resolution microscopes. Current tips can have a radius less than 400.ANG., and are square-pyramidal in shape. The multiple contact points on the tip can result in atomic size images which show a complex superposition of effects.
Some of the present microscopes employ tips made of tungsten, which tips are further coated with a layer of silicon. One drawback with such tungsten tips is the difficulty in depositing the silicon in the desired thinness.
Other tips for scanning tunneling microscopes are disclosed in U.S. Pat. No. 4,985,627 entitled, "Spin-Polarized Scanning Tunneling Microscope," and U.S. Pat. No. 4,968,585 entitled, "Microfabricated Cantilever Stylus with Integrated Conical Tip; Semiconductor Integrated Circuit Fabrication Techniques for Tip for Scanning Tunneling Microscope."
In contrast to the prior art, tips fabricated by the process of the present invention are approximately 7.ANG.-10.ANG. at the apex. The process of the present invention employs dry etching (also referred to as plasma etching) to fabricate sharp tips. Plasma etching is the selective removal of material through the use of etching gases. It is a chemical process which uses plasma energy to drive the reaction. Those factors which control the precision of the etch are the temperature of the etchant, the time of immersion, and the composition of the gaseous etchant.
Various papers refer to reactive ion etching (RIE) and orientation dependent etching (ODE) of silicon to form tips. These technologies rely on either expensive multiple deposition and evaporation steps, or dry etch processes bound by the isotropic etching characteristics of the process gases. For example, prior art dry etch processes limit the manufacturer to a height to width etch ratio of 1:1. To alter this 1:1 ratio in order to obtain an increased depth, a deeper mask would be required.